hotfix(sessionresolver): prevent data race inside http3 (#809)

See ooni/probe#2135 (comment)

internal/engine/internal/sessionresolver/childresolver.go

@@ -22,27 +22,16 @@ func timeLimitedLookup(ctx context.Context, re model.Resolver, hostname string)
 	return timeLimitedLookupWithTimeout(ctx, re, hostname, defaultTimeLimitedLookupTimeout)
 }
 
-// timeLimitedLookupResult is the result of a timeLimitedLookup
-type timeLimitedLookupResult struct {
-	addrs []string
-	err   error
-}
-
 // timeLimitedLookupWithTimeout is like timeLimitedLookup but with explicit timeout.
 func timeLimitedLookupWithTimeout(ctx context.Context, re model.Resolver,
 	hostname string, timeout time.Duration) ([]string, error) {
-	outch := make(chan *timeLimitedLookupResult, 1) // buffer
+	// In https://github.com/ooni/probe-cli/pull/807, I modified this code to
+	// run in a background goroutine and this resulted in a data race, see
+	// https://github.com/ooni/probe/issues/2135#issuecomment-1149840579. While
+	// I could not reproduce the data race in a simple way, the race itself
+	// seems to happen inside the http3 package. For now, I am going to revert
+	// the change causing the race and I'll investigate later.
 	ctx, cancel := context.WithTimeout(ctx, timeout)
 	defer cancel()
-	go func() {
-		out := &timeLimitedLookupResult{}
-		out.addrs, out.err = re.LookupHost(ctx, hostname)
-		outch <- out
-	}()
-	select {
-	case <-ctx.Done():
-		return nil, ctx.Err()
-	case out := <-outch:
-		return out.addrs, out.err
-	}
+	return re.LookupHost(ctx, hostname)
 }

internal/engine/internal/sessionresolver/childresolver_test.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"io"
 	"testing"
-	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/ooni/probe-cli/v3/internal/model/mocks"
@@ -43,25 +42,3 @@ func TestTimeLimitedLookupFailure(t *testing.T) {
 		t.Fatal("expected nil here")
 	}
 }
-
-func TestTimeLimitedLookupWillTimeout(t *testing.T) {
-	done := make(chan bool)
-	block := make(chan bool)
-	re := &mocks.Resolver{
-		MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
-			defer close(done)
-			<-block
-			return nil, io.EOF
-		},
-	}
-	ctx := context.Background()
-	out, err := timeLimitedLookupWithTimeout(ctx, re, "dns.google", 10*time.Millisecond)
-	if !errors.Is(err, context.DeadlineExceeded) {
-		t.Fatal("not the error we expected", err)
-	}
-	if out != nil {
-		t.Fatal("expected nil here")
-	}
-	close(block)
-	<-done
-}